Aminoplast resins have been used in a variety of applications such as in fire retardant compositions, glass fiber thermal insulation, plywood processing, molding composition and decorative lamination. Melamine-formaldehyde resins have been used as a crosslinking agent for the preparation of pressure sensitive adhesives and primers. For example, aqueous polymer dispersions are often used with said resins to enhance bonding characteristics. Several patents describing such adhesives and primers are described below.
Fitts et al., U.S. Pat. Nos. 5,075,360 and 5,187,210 disclose adhesives employed in manufacturing corrugated board. The adhesive composition comprises starch, boron containing compound, a basic agent and optionally, a water resistant additive comprising aminoplast resins such as urea formaldehyde, ketone formaldehyde and melamine formaldehyde.
Sheets et al., U.S. Pat. No. 5,059,264 discloses an aqueous dispersed primer-sealer composition applied to a surface layer of overlay paper or surface mat. The primer-sealer is based on a polymeric binder composition containing melamine resin, carboxyl functional acrylic emulsion copolymer, vinyl-acetate-vinyl versatate copolymer, emulsified fatty acid, and polyvinyl alcohol.
Pressure sensitive adhesives are used in a variety of applications due to their ability to form a bond at room temperature immediately upon contact, merely by the application of light pressure.
Tack, peel and shear are three properties which are useful in characterizing the nature of pressure sensitive adhesives. The tack measures the adhesive's ability to adhere quickly. Peel measures the ability of the adhesive to resist forced delamination. Shear measures the adhesive's ability to hold in position when shearing forces are exerted. Although tack and peel are directly related to each other, they are inversely related to shear.
A major disadvantage of pressure sensitive adhesives is that by chemical nature, they often have an inherently low cohesive strength. In addition, since they are thermoplastic and soften rapidly at a temperature in excess of about 140.degree. F. (60.degree. C.), they are limited to applications where the stress of the bond or assembled joint is low, i.e., generally less than 2 psi in shear.
Hoeschele et al., U.S. Pat. No. 4,234,500 and Traynor et al., U.S. Pat. No. 4,726,982 disclose improving loop tack and peel by formulating pressure sensitive adhesives with a tackifying resin. However, shear resistance could only be improved by sacrificing peel and loop tack in a single polymer composition or by formulating with a high shear product such as an elastomer or natural rubber latex. Thus, maintaining a requsite balance of poperties while improving one adhesive property, may detrimentally affect one or more of the desirable properties of pressure sensitive adhesives.
Sanderson et al., U.S. Pat. No. 4,077,926 disclose pressure sensitive adhesives containing polymerized alkoxyalkyl esters of unsaturated carboxylic acid. While the polymer of the invention is preferably an organic solution or less desirably an aqueous dispersion prepared in those media, it may also be used and/or prepared as a nonaqueous dispersion, a bulk polymerizable, an organic solvent solution of the polymer emulsified in water, and the like. In a preferred embodiment, an organic compound having two or more functional groups capable of reacting with the carboxylic and hydroxyl groups in the polymer is included to crosslink the said carboxylic acid groups. Among the polyfunctional compounds are phenol-formaldehyde, aminoplasts such as melamine-formaldehyde and alkylated melamine-formaldehyde. The amount of crosslinker is preferably from about 1% to about 5%. According to the invention, shear resistance is improved by crosslinking while retaining excellent tack.
A drawback with pressure sensitive adhesives described in the prior art references relates to shear strength. The pressure sensitive adhesives often have high tack, high peel strengh but lack adequate shear strength.
Therefore, there remains a need for water based pressure sensitive adhesives that exhibit excellent tack and peel strength with substantial improvements in shear strength when subjected to increased thermal and/or mechanical stress.